1. Field of the Invention
This invention relates to an output circuit device which detects signal charge transferred thereto from a charge transfer section of a charge transfer element such as a solid-state imaging element and converts the signal charge into a signal voltage.
2. Description of the Related Art
One of important characteristics required for recent CCD (charge coupled device) solid-state imaging elements is the conversion capacity for converting low signal charge into a signal voltage together with the structure of a sensor section in order to prevent possible deterioration of the sensitivity at a low illuminance when the imaging element is formed in a reduced size.
In order to raise the voltage conversion capacity of signal charge, the parasitic capacitance of the charge detection section must necessarily be reduced as low as possible. In order to reduce the parasitic capacitance, the capacitance between the gate and the channel of a NOS transistor which receives signal charge first should be reduced, that is, the area of the gate should be reduced.
However, if the area of the gate of a MOS transistor is reduced, then the 1/f noise which relies upon the area of the gate of a NOS transistor constituting an output circuit is increased.
Consequently, the picture quality at a low illuminance is deteriorated, which makes an obstacle to maintenance or enhancement of the signal to noise ratio at a level of conventional products when it is tried to minimize CCD solid-state imaging elements.
An output circuit section of a CCD imaging element is shown in FIGS. 10 and 11. Referring to FIGS. 10 and 11, a gate electrode 103 is formed on the surface of a semiconductor body 101 with a gate insulation film 102 interposed therebetween. A pair of source-drain regions 104 and 105 are formed on the upper face of the semiconductor body 101 on the opposite sides of the gate electrode 103.
A high concentration diffusion layer 106 is formed at a location on a low concentration diffusion layer 113 of a reset transistor 111, a horizontal output transistor 112 and so forth provided on the upper face of the semiconductor body 101 to which a wiring line 127, which will be hereinafter described, is connected. The high concentration diffusion layer 106 exhibits a floating condition with respect to the semiconductor body 101.
An interlayer insulating layer 121 is formed on the semiconductor body 101. The interlayer insulation film 121 has contact holes 122. 123 and 124 formed therein above the source-drain regions 104 and 105 and the high concentration diffusion layer 106, respectively. A wiring line 125 extends through the contact hole 122 and is connected to the source-drain region 104, and another wiring line 126 extends through the contact hole 123 and is connected to the source-drain region 105. A further wiring line 127 extends through the contact hole 124 and is connected to the high concentration diffusion layer 106.
The source-drain regions 104 and 105 and the high concentration diffusion layer 106 are formed by diffusion of arsenic, phosphor or a like element in a high concentration.
Where, for example, arsenic is diffused in a high concentration to form the source-drain regions 104 and 105 and the high concentration diffusion layer 106, the depth of the junction is small since the diffusion coefficient of arsenic is low.
Thus, since the depth of the junction is small with an output circuit section wherein a high concentration diffusion layer (hereinafter referred to as diffusion layer) such as the layer of the source-drain regions or the high concentration diffusion layer is formed by diffusion of arsenic in a high concentration, where an aluminum spike is produced at a junction between the diffusion layer and a wiring line, the aluminum spike penetrates through the diffusion layer to cause a junction leak. As a result, the reliability of the element is deteriorated significantly.
Further, in a CCD element of the aluminum shunt structure wherein a wiring line serves also as a light interception film, if an aluminum-silicon alloy is employed as a material for a wiring line in order to suppress production of an aluminum spike, then silicon is deposited in the inside of the aluminum-silicon alloy so that the wiring line becomes inclined to transmit light therethrough. Light having transmitted through the wiring line is admitted into the transfer electrode, resulting in deterioration of the smear characteristic. This makes a fatal defect to the CCD element. Consequently, the reliability of the CCD element is reduced significantly.
On the other hand, where phosphor is diffused in a high concentration to form a diffusion layer, since the phosphor is introduced to a very deep location of the semiconductor body, the diffusion layer is formed deep. Consequently, the subject of a junction leak arising from an aluminum spike is solved. However, since phosphor has a higher diffusion coefficient than arsenic, the diffusion layer is formed over a wide area also in radial directions in the semiconductor body. Consequently, in a MOS transistor, the diffusion layer exhibits a greater overlapping area with a gate electrode. As a result, the gate electrode is present on the n-type diffusion layer of a high concentration, and consequently, a depletion layer is not extended toward the semiconductor body. Consequently, the capacitance per unit area of the portion is increased, which significantly deteriorates the efficiency in conversion of charge into a voltage.
In this manner, where the depth of a diffusion layer is small, the subject of an aluminum spike is involved, but where the depth of a diffusion layer is great, the subject of an increase of the capacitance is involved.
Further, where the concentration of a diffusion layer is reduced, the contact resistance between a wiring line and the diffusion layer is increased, which leads to the subject that the driving capacity of the output circuit section is reduced.